Welding is a long practiced process for joining separate metal objects together for the purpose of building support structures and producing pipes, vessles and other products. There are many types of welding techniques including gas welding, electric arc welding and electric resistance welding (ERW). A common characteristic of welded objects is the difference in thickness between the welded and non-welded regions.
Inspection of weld integrity has long been recognized as crucially important because it enables the detection of cracking, porosity, incomplete penetration, inclusions, lack of sidewall fusion, and other defects that can compromise weld strength. Many existing NDT/NDI products are available for weld inspection, such as ultrasonic and eddy current instruments and probes.
For some weld inspection applications, one of the challenges lies in how to efficiently and/or automatically track the position of the welds and therefore guide the inspection probes that are placed in immediate proximity of the welds. For example, for electronic resistance welding (ERW) production, pipes are formed by feeding rectangular metal sheets into the welder to have their opposite sides welded together. Then the weld seam is trimmed on both outer and inner surfaces before leaving the welding and weld verification system.
In one of the existing practices, the weld seam integrity is inspected while being transported through the welding and weld inspection system by an ultrasonic phased array (PA) system. The weld seam position on long pipes has a tendency to drift circumferentially while in transport requiring an operator to visually monitor the weld seam position and adjust the pipe or the PA probe position to remain centered over the weld seam. It should be noted that it is not uncommon for the weld seam location to drift only a few degrees for every 50 meter of pipe's axial movement.
One of the drawbacks of the manual tracking method described above is that it prevents the full automation of the welding and weld inspection system because the operator must be present to monitor and adjust the weld seam location as required.
To overcome the problem of weld seam tracking, some existing efforts came to the applicants' knowledge including a solution provided by Metavision of the United Kingdom. The Metavision device uses laser tracking technology to guide NDI inspection. A limitation presented by this method is that it requires a visible marked line along the pipe to guide the PA probe to follow the weld seam. Painting a visible line (called also the pilot line) at a given distance from the weld seam presents quite some degree of difficulties under industrial operation conditions. In order to be precise, the pilot line has to be painted near the welding process location where the temperature is hot, consequently causing the marked line to lack paint or be partially masked. The laser tracking device, an optical device, is used to monitor the contrast between the dark surface on the pipe and the white line to establish proper position. If the contrast is poor, the device could lose the weld position.
Additional disadvantages of the Metavision device also include the use of a laser sensor and the need for two separate sets of sensors and associated hardware/software for weld seam tracking and inspection tasks, which leads to undesirable higher system investment.
Another solution is described in US patent application (publication number 2009/0132181 A1) by Girndt. The Girndt patent application addresses the weld seam tracking problem by employing an ultrasonic sensor to track the weld seam using two sets of sensors—one for tracking and the other for inspecting the weld seam, which presents increased material and operational costs.
Girndt also fails to mention the usage of phased array ultrasonic system, which prevents obtaining the capability to cover a wider stripe of scanning area along weld seam and its vicinity. Furthermore, Girndt presents the limitation of requiring the scanning of the whole external pipe surface to obtain the weld seam position, which is extremely inefficient compared to scanning only the weld seam and the bordering region.